Kansas governor Sam Brownback apologised Monday for his office's response to a disparaging Twitter post by a high school senior who was attending a school function at the state capitol.

"My staff overreacted to this tweet, and for that I apologise," Brownback said in a Monday statement. "Freedom of speech is among our most treasured freedoms."

Emma Sullivan, 18, of the Kansas City suburb of Fairway, was taking part in a Youth in Government program last week in Topeka when she sent out a tweet from the back of a crowd of students listening to Brownback's greeting. From her cellphone, she thumbed: "Just made mean comments at gov. brownback and told him he sucked, in person #heblowsalot."

She said she was just joking with friends, but Brownback's office, which monitors social media for postings containing the governor's name, contacted the youth program. Sullivan said she spent nearly an hour in the principal's office and was told to apologise in writing to the governor.

As word of the governor's office reaction spread, Sullivan went from 61 Twitter followers before the incident to more than 9,000 Monday - more than three times the number that follow Brownback's official Twitter account.

The Shawnee Mission East senior decided to not write the apology letter and the school district issued a statement Monday saying there would be no repercussions.

"Whether and to whom any apologies are issued will be left to the individuals involved," the statement said. "The issue has resulted in many teachable moments concerning the use of social media. The district does not intend to take any further action on this matter."

Doug Bonney, legal director for the American Civil Liberties Union of Kansas and Western Missouri, said the teen's speech was clearly protected by the First Amendment.

"Saying that the governor is no good and is a blowhard is core protected speech," Bonney said. "It's absolutely what the First Amendment was designed to protect."

Sullivan's 19-year-old sister, Olivia, said her sister was in school Monday when their father talked to school officials. The elder sister, who initially alerted the media about what happened, said she texted with her sister and the teen told her "things were fine."

Emma Sullivan said Sunday that she thought the tweet "has turned into a good starting point to open up dialogue about this ... free speech and the power of social media and the power that people my age could potentially have, that people will listen to us."